Shareholders' Equity - Stock options transactions (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Outstanding stock options [Roll forward]
Beginning balance	2,568,000
Granted	0
Exercised	(483,195)	(351,973)	(285,938)
Forfeited	(20,000)
Ending balance	2,065,000	2,568,000
Vested and expected to vest	2,065,000
Options exercisable	1,640,000
Weighted-average exercise price [Abstract]
Beginning balance	$ 60.04
Ending balance	62.08	$ 60.04
Vested and expected to vest	62.08
Options exercisable	$ 53.40
Weighted-Average Remaining Term	4 years
Weight-Average remaining term vested and expected to vest	4 years
Weighted Average remaining term options exercisable	3 years 6 months
Aggregate intrinsic value, outstanding	$ 42,437
Aggregated intrinsic value, vested and expected to vest	42,437
Aggregate intrinsic value, exercisable	$ 34,450